View source: R/blueprintformuladefault.R
default_formula_blueprint  R Documentation 
This pages holds the details for the formula preprocessing blueprint. This
is the blueprint used by default from mold()
if x
is a formula.
default_formula_blueprint(
intercept = FALSE,
allow_novel_levels = FALSE,
indicators = "traditional",
composition = "tibble"
)
## S3 method for class 'formula'
mold(formula, data, ..., blueprint = NULL)
intercept 
A logical. Should an intercept be included in the
processed data? This information is used by the 
allow_novel_levels 
A logical. Should novel factor levels be allowed at
prediction time? This information is used by the 
indicators 
A single character string. Control how factors are expanded into dummy variable indicator columns. One of:

composition 
Either "tibble", "matrix", or "dgCMatrix" for the format of the processed predictors. If "matrix" or "dgCMatrix" are chosen, all of the predictors must be numeric after the preprocessing method has been applied; otherwise an error is thrown. 
formula 
A formula specifying the predictors and the outcomes. 
data 
A data frame or matrix containing the outcomes and predictors. 
... 
Not used. 
blueprint 
A preprocessing 
While not different from base R, the behavior of expanding factors into
dummy variables when indicators = "traditional"
and an intercept is not
present is not always intuitive and should be documented.
When an intercept is present, factors are expanded into K1
new columns,
where K
is the number of levels in the factor.
When an intercept is not present, the first factor is expanded into
all K
columns (onehot encoding), and the remaining factors are expanded
into K1
columns. This behavior ensures that meaningful predictions can
be made for the reference level of the first factor, but is not the exact
"no intercept" model that was requested. Without this behavior, predictions
for the reference level of the first factor would always be forced to 0
when there is no intercept.
Offsets can be included in the formula method through the use of the inline
function stats::offset()
. These are returned as a tibble with 1 column
named ".offset"
in the $extras$offset
slot of the return value.
For default_formula_blueprint()
, a formula blueprint.
When mold()
is used with the default formula blueprint:
Predictors
The RHS of the formula
is isolated, and converted to its own
1 sided formula: ~ RHS
.
Runs stats::model.frame()
on the RHS formula and uses data
.
If indicators = "traditional"
, it then runs stats::model.matrix()
on the result.
If indicators = "none"
, factors are removed before model.matrix()
is run, and then added back afterwards. No interactions or inline
functions involving factors are allowed.
If indicators = "one_hot"
, it then runs stats::model.matrix()
on the
result using a contrast function that creates indicator columns for all
levels of all factors.
If any offsets are present from using offset()
, then they are
extracted with model_offset()
.
If intercept = TRUE
, adds an intercept column.
Coerces the result of the above steps to a tibble.
Outcomes
The LHS of the formula
is isolated, and converted to its own
1 sided formula: ~ LHS
.
Runs stats::model.frame()
on the LHS formula and uses data
.
Coerces the result of the above steps to a tibble.
When forge()
is used with the default formula blueprint:
It calls shrink()
to trim new_data
to only the required columns and
coerce new_data
to a tibble.
It calls scream()
to perform validation on the structure of the columns
of new_data
.
Predictors
It runs stats::model.frame()
on new_data
using the stored terms
object corresponding to the predictors.
If, in the original mold()
call, indicators = "traditional"
was
set, it then runs stats::model.matrix()
on the result.
If, in the original mold()
call, indicators = "none"
was set, it
runs stats::model.matrix()
on the result without the factor columns,
and then adds them on afterwards.
If, in the original mold()
call, indicators = "one_hot"
was set, it
runs stats::model.matrix()
on the result with a contrast function that
includes indicators for all levels of all factor columns.
If any offsets are present from using offset()
in the original call
to mold()
, then they are extracted with model_offset()
.
If intercept = TRUE
in the original call to mold()
, then an
intercept column is added.
It coerces the result of the above steps to a tibble.
Outcomes
It runs stats::model.frame()
on new_data
using the
stored terms object corresponding to the outcomes.
Coerces the result to a tibble.
There are a number of differences from base R regarding how formulas are
processed by mold()
that require some explanation.
Multivariate outcomes can be specified on the LHS using syntax that is
similar to the RHS (i.e. outcome_1 + outcome_2 ~ predictors
).
If any complex calculations are done on the LHS and they return matrices
(like stats::poly()
), then those matrices are flattened into multiple
columns of the tibble after the call to model.frame()
. While this is
possible, it is not recommended, and if a large amount of preprocessing is
required on the outcomes, then you are better off
using a recipes::recipe()
.
Global variables are not allowed in the formula. An error will be thrown
if they are included. All terms in the formula should come from data
. If
you need to use inline functions in the formula, the safest way to do so is
to prefix them with their package name, like pkg::fn()
. This ensures that
the function will always be available at mold()
(fit) and forge()
(prediction) time. That said, if the package is attached
(i.e. with library()
), then you should be able to use the inline function
without the prefix.
By default, intercepts are not included in the predictor output from the
formula. To include an intercept, set
blueprint = default_formula_blueprint(intercept = TRUE)
. The rationale
for this is that many packages either always require or never allow an
intercept (for example, the earth
package), and they do a large amount of
extra work to keep the user from supplying one or removing it. This
interface standardizes all of that flexibility in one place.
# 
data("hardhatexampledata")
# 
# Formula Example
# Call mold() with the training data
processed < mold(
log(num_1) ~ num_2 + fac_1,
example_train,
blueprint = default_formula_blueprint(intercept = TRUE)
)
# Then, call forge() with the blueprint and the test data
# to have it preprocess the test data in the same way
forge(example_test, processed$blueprint)
# Use `outcomes = TRUE` to also extract the preprocessed outcome
forge(example_test, processed$blueprint, outcomes = TRUE)
# 
# Factors without an intercept
# No intercept is added by default
processed < mold(num_1 ~ fac_1 + fac_2, example_train)
# So, for factor columns, the first factor is completely expanded into all
# `K` columns (the number of levels), and the subsequent factors are expanded
# into `K  1` columns.
processed$predictors
# In the above example, `fac_1` is expanded into all three columns,
# `fac_2` is not. This behavior comes from `model.matrix()`, and is somewhat
# known in the R community, but can lead to a model that is difficult to
# interpret since the corresponding pvalues are testing wildly different
# hypotheses.
# To get all indicators for all columns (irrespective of the intercept),
# use the `indicators = "one_hot"` option
processed < mold(
num_1 ~ fac_1 + fac_2,
example_train,
blueprint = default_formula_blueprint(indicators = "one_hot")
)
processed$predictors
# It is not possible to construct a nointercept model that expands all
# factors into `K  1` columns using the formula method. If required, a
# recipe could be used to construct this model.
# 
# Global variables
y < rep(1, times = nrow(example_train))
# In base R, global variables are allowed in a model formula
frame < model.frame(fac_1 ~ y + num_2, example_train)
head(frame)
# mold() does not allow them, and throws an error
try(mold(fac_1 ~ y + num_2, example_train))
# 
# Dummy variables and interactions
# By default, factor columns are expanded
# and interactions are created, both by
# calling `model.matrix()`. Some models (like
# tree based models) can take factors directly
# but still might want to use the formula method.
# In those cases, set `indicators = "none"` to not
# run `model.matrix()` on factor columns. Interactions
# are still allowed and are run on numeric columns.
bp_no_indicators < default_formula_blueprint(indicators = "none")
processed < mold(
~ fac_1 + num_1:num_2,
example_train,
blueprint = bp_no_indicators
)
processed$predictors
# An informative error is thrown when `indicators = "none"` and
# factors are present in interaction terms or in inline functions
try(mold(num_1 ~ num_2:fac_1, example_train, blueprint = bp_no_indicators))
try(mold(num_1 ~ paste0(fac_1), example_train, blueprint = bp_no_indicators))
# 
# Multivariate outcomes
# Multivariate formulas can be specified easily
processed < mold(num_1 + log(num_2) ~ fac_1, example_train)
processed$outcomes
# Inline functions on the LHS are run, but any matrix
# output is flattened (like what happens in `model.matrix()`)
# (essentially this means you don't wind up with columns
# in the tibble that are matrices)
processed < mold(poly(num_2, degree = 2) ~ fac_1, example_train)
processed$outcomes
# TRUE
ncol(processed$outcomes) == 2
# Multivariate formulas specified in mold()
# carry over into forge()
forge(example_test, processed$blueprint, outcomes = TRUE)
# 
# Offsets
# Offsets are handled specially in base R, so they deserve special
# treatment here as well. You can add offsets using the inline function
# `offset()`
processed < mold(num_1 ~ offset(num_2) + fac_1, example_train)
processed$extras$offset
# Multiple offsets can be included, and they get added together
processed < mold(
num_1 ~ offset(num_2) + offset(num_3),
example_train
)
identical(
processed$extras$offset$.offset,
example_train$num_2 + example_train$num_3
)
# Forging test data will also require
# and include the offset
forge(example_test, processed$blueprint)
# 
# Intercept only
# Because `1` and `0` are intercept modifying terms, they are
# not allowed in the formula and are instead controlled by the
# `intercept` argument of the blueprint. To use an intercept
# only formula, you should supply `NULL` on the RHS of the formula.
mold(
~NULL,
example_train,
blueprint = default_formula_blueprint(intercept = TRUE)
)
# 
# Matrix output for predictors
# You can change the `composition` of the predictor data set
bp < default_formula_blueprint(composition = "dgCMatrix")
processed < mold(log(num_1) ~ num_2 + fac_1, example_train, blueprint = bp)
class(processed$predictors)
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